* mostly restored the tiled importer unit tests
* code cleanup
* code cleanup
* almost got tiled maps serialization independent of the content pipeline
* refactored the tiled map content importer so that tiled maps can be loaded at runtime
* code cleanup
* removed nuspec files
* experimenting with an XML based GUI markup parser
* a few more markup parser features
* finally got a working open file dialog
* experimenting with markup bindings
* attached properties
* working on the gui stuff again
* working on a better textbox
* multiline textboxing
* new text box is really coming along
* removed the content explorer experiment
* restored the old gui demo back to the way it was before (for now)
`FirstGlobalIdentifier` is only used with the map to reference the tileset which contains the tile referenced. It isn't really part of the definition of a `TiledMapTileset` even though the `.TMX` format includes it with the `<tileset>` tag
Pulled out the methods for writing and reading the content files for tilesets into their own classes to allow loading tilesets independently from the maps.
Dylan Wilson